In a conventional multi-stage centrifugal compressor, a long shaft is required to drive the corresponding plurality of impellers. A long shaft limits the speed at which the compressor can be operated, and causes vibration problems. Impellers cannot exceed a certain speed limit for a given impeller tip diameter because this speed limit is set by aerodynamic considerations. However, when impellers are mounted on a shaft, the impeller tip diameters have to be increased to compensate for the diameter of the shaft, thereby limiting the speed at which the impellers can be operated.
For a given rotational speed of a conventional compressor, only a limited number of impellers can be mounted on a single shaft. While this problem can be avoided in a particular application by employing a plurality of multi-stage centrifugal compressors, the cost of the system is substantially increased.
The conventional centrifugal compressor also requires seals at both ends of the shaft, as the drive for the compressor is located exteriorly of the compressor housing. As an external driver drives all of the impellers in a single centrifugal compressor through a single shaft, a custom coupling is needed to satisfy high torque requirements, thereby increasing the costs of the system.